The present disclosure relates to field effect transistors (FETs), and more particularly to transistors using nitrides, which are used for inverters, power supply circuits, etc.
Group III-V nitride compound semiconductors such as gallium nitride (GaN), which are generally called nitride semiconductors, have received attention. The nitride semiconductors are compound semiconductors that are formed by aluminum (Al), gallium (Ga), and indium (In) as group III elements and nitrogen (N) as a group V element, and is represented by the general formula InxGayAl1-x-yN (where 0≦x≦1, 0≦y≦1, and x+y≦1). The nitride semiconductors can form various alloys, and can easily form a heterojunction interface. The heterojunctions of the nitride semiconductors are characterized in that a high density two-dimensional electron gas (2DEG) layer is formed near the junction interface due to spontaneous polarization or piezoelectric polarization even in a non-doped state. FETs using this high density 2DEG layer as carriers have received attention as high frequency and high power devices.
However, a phenomenon called “current collapse” tends to occur in the FETs using nitride semiconductors. The current collapse is a phenomenon in which a current decreases for a certain period of time when the device is turned on again after being turned off. Poor characteristics regarding the current collapse make high speed switching operations difficult, causing a very serious problem in the device operation.
One method to reduce the current collapse is to form a protective film on the surface of an electron supply layer. An example of the protective film is a silicon nitride film (a SiN film) or a p-type organic semiconductor film (see, e.g., Japanese Patent Publication No. 2007-27284).